Semiconductor devices such as transistors and integrated circuits are typically formed on a substrate of a semiconducting material, using processes such as etching, lithography, and ion implantation to form various structures and materials on the substrate. A single field-effect transistor (FET), for example, may require a dozen or more steps to form implanted source and drain regions, an insulating layer, and a gate separated from the channel region by the insulating region.
In operation, doped source and drain regions are coupled to a circuit such that a voltage signal applied to the gate region controls the conductivity or resistivity of a channel region physically located between the source and drain regions. The conductivity of the channel region is based on an electric field created by potential applied to the gate, relative to the voltages present at the source and drain. Field effect transistors are sometimes described as being voltage-controlled resistors for this reason, and are used for applications such as amplifiers, signal processing, and control systems.
Field effect transistors are also very common in digital logic circuits such as in computer processors, memory, and other digital electronics. The voltage applied to the gate in such applications is typically intended to either turn off the FET completely or turn it on completely, such that the FET operates more like a switch than a variable resistor. For such applications, the switching speed, device size, leakage current, and a variety of other parameters are designed to provide the desired device size and operating characteristics, within the limitations of available technology. It is therefore desirable to control various parameters of field effect transistors to produce field effect transistors suited for various applications.